In conventional storage of data on optical storage media, such as on compact disc read only memory (CD-ROM) and DVD-RAM, a selected form of modulation encodes data into the surface of the media. In the context of DVD-ROM or DVD-RAM, an eight-fourteen modulation (efm) scheme is used to encode binary data through data “pits” that are either magnetically or optically inscribed within, or manually embossed/stamped on, the surface of the optical storage medium and undisturbed mirror regions. The length of the pit or complementary mirror is indicative of the encoded binary information, subject to there being no defects associated with the formation of the pit or mirror.
In a DVD-RAM, data segments (or sectors) spiral outward from a center of the optical storage medium. The data segments are also indexed by a header that is embossed (i.e., physically stamped) onto the surface of the optical storage medium. The header provides address and location information, such as track and sector numbers. The headers are individually indexed at the beginning of the disc for use in scanning. The headers have a precisely defined width dimension and are separated by a data sector of defined length. Furthermore, the headers appear in pairs that are physically offset from one another relative to a central datum within each track. Each pair of headers is generally followed by an extended mirror region of maximum reflectivity.
From a perspective of data recovery, once on-track, an array of photodiodes, typically four, is used to recover the information stored on the medium. The four photodiodes provide an output voltage that varies according to an amount of reflectivity from the surface of the medium. More particularly, laser light is reflected from the marks and spaces, with a data pit (i.e., a mark) providing an inferior reflectivity and hence a lower voltage than a space (that provides maximum reflectivity and hence maximum voltage).
Data encoding for DVD-RAM is further complicated by the structure employed within the readable/writeable medium. In addition to the spiraling and sectorized nature of the modulation data, the marks and spaces are produced within adjacent “lands” and “grooves” that provide a distinct three-dimensional profile to a cross-section of the optical storage medium. The lands and grooves also constitute “tracks” within the storage medium. Moreover, the lands and grooves exhibit a sinusoidal oscillation known as “wobble” which has a frequency of about 157 kHz at a 1×rate, which is typically lower than the rate of the efm data. The wobble, which is stamped into the optical storage medium, provides speed of rotation information that is critical for operation control of data read and data write functions. A frequency of the wobble is implemented for phase acquisition in a phase lock loop. More specifically, the wobble frequency provides a synchronized write clock having a known linear density of information. Unfortunately, the embossed header regions entirely disrupt (or break) the continuous sinusoidal oscillation of the wobble. The wobble simply does not co-exist with header regions. The loss of the wobble signal adversely affects phase acquisition, PLL function and device operation.
In contrast with efm data extraction that takes a sum of the four photodiodes, wobble extraction utilizes a “push-pull” signal obtained from the numeric subtraction of adjacent photodiode levels, namely the algebraic expression (A+B)−(C+D) where A, B, C and D are a sequence of adjacent photodiodes in a configuration of a square. For completeness, low pass filtering of the push-pull signal eliminates high frequency noise, such as produced from read frequency feed-through and any mismatches in, for example, amplification paths, to produce an appropriate signal from which a clock can be derived. As will be understood, in the ideal case, read frequency errors should be negligible (if not zero). However, in practice errors are induced by inaccuracies in the laser and detector alignment with respect to a center of a particular track on the optical medium. Low frequency noise is generally present as a consequence of introduction by processing and physical properties such as non-planar disc profiles, disc eccentricity, changes in reflectivity and errors in the servo-drive system for control of the laser and detector heads. Existing systems that utilize low pass filters for wobble recovery are unable to filter out such low pass noise, since the wobble signal is relatively dirty and interpretational errors may be induced.
As will be understood, when the array of photodiodes encounters each associated pair of headers, the photodiodes produce a maximum or minimum DC deflection (relative to efm data) in the push-pull signal. Moreover, the headers produce an indeterminate number spikes within the push-pull signal. A transition between each header in the pair also generates a reversal in the DC deflection. The relative polarity between spikes in the bandpass filtered push-pull signal caused by the headers provides an indication on whether a succeeding data sector appears on a land or a groove. The polarity information is necessary to instruct the PLL to perform a phase inversion. For completeness, it will be understood that the phase inversion (i.e., a 180° phase shift) always occurs at a transition between data sectors on lands and grooves.
In summary, DVD-RAM read/write operations require a clock to be generated which is phase lock ed to the wobble signal derived from a readback (or read channel) signal produced by spinning of the optical disc. The wobble is derived from an eccentricity deliberately produced in the track structure of DVD-RAM discs. The eccentric wobble is, however, not continuous and is broken up by embossed header regions (containing track addressing information). Therefore, phase locked loops (PLLs) attempting to lock onto the wobble signal are generally subject to loss of lock during header periods.